This invention relates to a wafer processing machine. More particularly, this invention relates to a processing unit for working the peripheral edge of a wafer used in the semiconductor industry. Still more particularly, this invention relates to a processing unit which can be used to polish the peripheral edge of a wafer or to remove edge bead material from the edge of a substrate such as a processed wafer.
As is known, various types of wafers, such as silicon wafers, have been employed in the manufacture of semi-conductor chips. Typically, the wafers have been obtained by the slicing of a solid cylindrical ingot into individual wafers. Once cut, the wafers are processed in various manners and particularly to provide a peripheral edge of a predetermined contour. Various types of grinding machines have been employed for this purpose.
During the processing of a wafer into semi-conductor chips, it has been found that small sub-surface cracks or fractures at the peripheral edge of a wafer have a tendency of migrating into the wafer to such an extent that a significant portion of the wafer becomes unusable for the manufacture of the semi-conductor chips. Accordingly, it has become important to avoid the occurrence of cracks at the outer periphery of a wafer and particularly cracks which have a tendency of migrating into the wafer during subsequent processing.
It has also been known that processed wafers which have a film of material formed thereon, as by a spinning technique, usually have an edge bead of the material formed along the peripheral edge. As described in U.S. Pat. Nos. 4,510,176; 4,732,785; 5,444,921 and 5,618,380 several techniques have been described for removing the edge bead of material. Other techniques have also been described, for example, in U.S. Pat. Nos. 5,398,372 and 5,702,537 for removing an edge bead from a side edge of a strip of material.
Still further, it has been known that when applying several layers of material to a wafer, that each layer may thin at the peripheral edge of the wafer causing a subsequent flaking problem.
Accordingly, it is an object of the invention to provide a relatively simple technique for polishing the peripheral edge of a ground wafer to a high degree of polish to minimize fracture depth.
It is another object of the invention to provide a relatively simple polishing unit for the polishing of the peripheral edge of a wafer.
It is another object of the invention to provide a processing machine which can be used to remove an edge bead from a processed wafer or any other substrate.
It is another object of the invention to provide a relatively simple polishing unit for wafers which can be retro-fitted onto an existing wafer processing machines.
It is another object of the invention to provide a compact grinding/polishing machine for the processing of wafers for the semi-conductor industry.
Briefly, the invention provides a processing unit for working an edge of a substrate. The unit includes a first means for positioning a working medium against an edge of a moving substrate and a second means for moving the working medium an a plane perpendicular to the substrate during movement of the substrate to place the working medium in contact with at least one side of the moving substrate.
The processing unit may be employed as a polishing unit with the working medium for polishing the peripheral edge of a wafer or may be employed to remove an edge bead of material on the peripheral edge of a processed wafer or of a strip of material. In the one case, the working medium would be a polishing medium and, in the other cases, the working medium would be a grinding medium or any other suitable material for removing material from the substrate.
In one embodiment, the processing unit is constructed as a wafer edge polishing unit for polishing a peripheral edge of a wafer which is located on a rotating chuck. This polishing unit includes a first means for positioning at least one polishing medium against the peripheral edge of a wafer on the chuck as well as a second means for moving the polishing medium in a plane perpendicular to the chuck during rotation of the chuck in order to place the polishing medium in contact with at least one side of the wafer on the chuck.
In accordance with the invention, the means for positioning the polishing medium includes an elongated backing block facing the chuck, a pair of spools and at least one tape having the polishing medium thereon wound on and extending between the spools and over the backing block. In addition, a clamping means is provided, for example, in the form of a pair of clamps on opposite sides of the block, for releasably clamping the tape to the sides of the block.
The means for positioning the polishing medium is mounted in a stationary manner relative to the wafer mounting chuck so that the chuck is movable towards and away from the tape on a Y-axis. However, the means for positioning the polishing medium may also be mounted to move relative to the chuck along the Y-axis.
An elongated facing plate is movably mounted on the block and is disposed in backing contact with the tape in order to provide a rigid surface for holding the tape against the edge of a wafer being polished. In addition, a sensing means is provided for sensing movement of the facing plate in response to contact of the tape with a wafer on the chuck and for emitting a responsive signal thereto as a measure of the contact force between the tape and the wafer on the chuck.
Where the facing plate is made of a rigid material, an elastomeric layer is also provided to mount the rigid facing plate thereon. This elastomeric layer serves to cushion the contact force between the rigid facing plate and the wafer.
The sensing means which is employed for sensing the movement of the facing plate and, thus, the tape relative to the wafer, includes a beam which mounts the facing plate thereon, a pair of load cells which support the beam at two ends and which emit corresponding signals in reaction to movement of the beam and a read-out connected to the load cells to display a numeric indication of the signals.
The means for moving the polishing medium perpendicularly of the chuck is constructed to pivot the block on which the tape is mounted about a pivot axis which passes longitudinally of a face of the block and which is disposed in a plane of the wafer on the chuck. The pivoting action is such as to move the tape between a first position on one side of the chuck and a second position on the opposite side of the chuck.
During operation, the polishing unit is usually stationary and the chuck for holding the wafer is movable relative to the polishing unit. To this end, after a wafer has been centered on the chuck, the chuck is moved towards the polishing unit until the edge of the wafer contacts the polishing medium. During this time, the block on which the tape with the polishing medium is mounted is pivoted to bring the tape into a position for line contact with one side of the edge of the wafer, e.g. a top bevel surface. After contact is made, the wafer then rotates or continues to rotate so that the polishing medium on the tape is able to polish the entire circumferential extent of the top bevel surface of the wafer.
The polishing unit is programmed so that the block on which the tape is mounted is pivoted or otherwise moved in a plane perpendicular to the wafer so that the tape follows the contour of the edge of the wafer and is then brought into line contact with the opposite side of the edge of the wafer, e.g. a bottom bevel surface.
Typically, wafers are ground to have a peripheral edge with a contour of trapezoidal shape (i.e. a I-type) or with a rounded apex (i.e. an R-type) on a trapezoidal shape. During polishing, the polishing medium follows this contour to polish the surface of the shaped edge to a high finish.
In a preferred embodiment, the polishing unit is provided with a plurality of polishing mediums with each having a different grade of abrasive from the other. For example, the first polishing medium may have a large diamond grit while the last polishing medium in the series has a fine diamond grit.
In the preferred embodiment, four tapes having diamond grits of different grade are mounted in the polishing unit in parallel side-by-side relation. Typically, the wafer is brought into contact with the first tape of the series in order to have a course polishing operation conducted while the wafer is being rotated. Thereafter, the chuck on which the wafer is mounted is indexed laterally of the polishing unit in order to bring the next tape into contact with the peripheral edge of the wafer to perform a further polishing cycle. The wafer is indexed in a similar fashion until polishing by the last tape has been performed.
Upon completion of a polishing operation, the peripheral edge of the wafer has been provided with a mirror-like high grade finish.
The polishing unit can be readily incorporated into a grinding machine or retro-fitted into an existing grinding machine.
Typically, the polishing unit would be disposed in a machine having a grinding stage for grinding a wafer to a predetermined diameter and a conveyor for moving a wafer from the grinding station to the polishing station. After a wafer has been polished in the polishing unit, the same conveyor may be used to convey the polished wafer to a delivery point for mounting in a cassette or onto another conveyor for transportation to another processing unit.
The polishing unit may also be incorporated into a spin/rinse/dry station. In this embodiment, after a wafer has been ground and before being rinsed and dried, the wafer may be polished in the same station that would subsequently rinse and spin dry the wafer. This avoids the need to transfer the polished wafer to a rinse station.
As an alternative, a second conveyor may be positioned within the machine to transfer the polished wafer to a delivery point without interfering with the conveyor used to transfer the ground wafer to the polishing station. Such conveyors may operate in a parallel arrangement so as to limit the space required for the conveyors.
In other embodiments, the processing unit may be suitably adapted to work the edge of a processed wafer to remove an edge bead of material as by grinding or to work the edge of a moving strip to remove an edge bead of material therefrom.